I'm a bit confused too. But according to urbandictionary, "flower pot" means "When you soil/shart you underwear, then remove the underwear and put them on someones head." Maybe the person who posted the fml meant soiled underwear? Even then its a bit confusing...unless the flatmate had sh*t themselves and came out in their underwear to help.

sarcasm is a foreign language to you, isn't it?